Screws of the aforesaid type have been known for some time, such as the classical screws with a cross recess known as "Philips" or "Pozidrive", and those known as "Torx" or "Tips". When produced on modern machines at a high production rate (up to 800 screws per minute), Philips and Pozidrive screws in particular, known for many decades, give rise to a series of problems due to the ease with which the equipment (punch) used to cold-form the recess in the screw head breaks, this resulting in considerable production slow-down.
After considerable study and experiment it has been concluded that this drawback is due particularly to the fact that the punch comprises fairly thin parts which easily break when the punch is subjected to the considerable number of blows per unit of time as required by current high-production machines. For similar reasons the tip of the relative screwdriver or screwing tool is subject to rapid wear, with the like possibility of breakage of some of its parts.
These known screws also suffer from the not inconsiderable drawback that a significant axial thrust has to be exerted by the relative screwdriver or motorized screwing tool when screwing them, in order to prevent the relative tip escaping from the recess in the screw. The axial thrust required for their screwing can even reach a level such as to deform the parts into which the screw is inserted if these are not sufficiently robust (for example in the case of laminations).
The Torx or Tips screws do not suffer from the drawback of escape of the screwing tip from the screw recess. In this respect such screws--the recess of which has an undulated star configuration in plan view--have the lateral wall of their recess formed by a cylindrical surface the directrix of which is of star shape and the generatrices are parallel to the screw axis. In addition the configuration of the recess--and hence of the punch used to form the recess, and of the appropriate screwing tool tip for the particular screw--is fairly solid, and hence able to resist the high working rate of currently available machines. However the relative punch is rather costly to construct because of its particular curved-surface configuration. In addition the fact that the recess is not flared complicates the cold-forming of the recess, to substantially reduce the number of screws obtainable per unit of time compared with the maximum number theoretically obtainable. The result is that the cost of such screws is considerably higher than that of screws with a traditional recess.